I think that this is a great idea Eugene. In fact I know of a couple of koi dealers that are using this method. For bigger koi they have a gigantic half-tub. I would definitely like to try this sometimes.
I myself use a fiberglass measuring tub. When I first started taking pictures the koi would just swim along the edges of the tub. Then I went to a dealer in Seattle and had the opportunity to observe the methods that they were using: they had a fiberglass measuring tub sitting on top of 8x8x16 blocks to make it higher. It's a two-people effort where one person would be the photographer and the other person would hold what looks to me like a small fiberglass rod. This person would use the rod to gently nudge the koi so that it would swim to the middle of the tank. I copied this method and had my wife help me out. I think that it was much easier and the pixs turned out better.
I use a Nikon D70 with a polarizer and a blue measuring tub for mine. I also like to take pixs in a semi-dark environment like my garage. It's definitely a lot of work but it's nice to be able to observe your koi up close.